1. Field of the Invention
This invention relates to the production of improved lubricating oils. In particular, it relates to a method of preparing stable lubricating oils which are highly resistant to oxidation and sludge formation when exposed to a highly oxidative environment. More particularly, it relates to a method of stabilizing hydroprocessed lube stocks by the addition of hydrogen sulfide to the hydrogen feed, without significantly increasing catalyst aging rate.
2. Description of Related Art
It is well known that certain types of organic compounds are normally susceptible to deterioration by oxidation or by corrosion through coming into contact with various metal surfaces. For example, it is known that liquid hydrocarbons in the form of fuels or lubricating oils tend to accumulate considerable quantities of water when maintained for long periods of time in storage vessels; and when subsequently brought into contact with metal surfaces in their functional environments, deterioration as a result of corrosion occurs. As a further example, in modern internal combustion engines and in turbojet engines, lubricants can be attacked by oxygen or air at high temperatures to form heavy viscous sludges, varnish and resins which become deposited on the engine surfaces. As a result, the lubricant cannot perform its required task effectively, and the engine does not operate efficiently. Furthermore, the sludges produced by lubricant deterioration generated by insufficient oxidative stability tend to foul and plug low tolerance hydraulic system components and interconnecting piping and valves. In addition, where such lubricating oils or other corrosion-inducing materials are incorporated into solid lubricants as in the form of greases, similar results are encountered, thus clearly indicating the necessity for improved methods of treatment which increase the oxidative stability of lubricating oils.
Accompanying the deterioration of lubricants by oxidation is the resultant corrosion of the metal surfaces for which such lubricants are designed and supplied. Once a lubricant has been oxidized to produce viscous sludges and resins, acids develop which are corrosive enough to destroy most metals. Moreover, the friction between metal parts increases following lubricant breakdown due to oxidation and leads to excessive metal wear. Increasing demands on lubricants, brought about by the widespread introduction of engines operating at steadily increasing temperatures, pressures, and speeds, necessitate a constant search for new methods of hydrocarbon treatment which can provide lubricants with increased oxidation resistance.
Due to the lubricant oxidative stability requirements for newer engines and other rotating or moving equipment lubrication, feedstocks which were previously suitable for lubricant production are presently unsuitable or at best marginal for such uses. Thus at a time when overall lubricant demands are increasing, the amount of suitable lubricant feedstock material is being diminished due to the oxidative stability requirements of newer machinery.
In order to produce lube stocks of suitable pour point from paraffin-containing feedstocks, it is generally necessary to remove significant amounts of hydrocarbon waxes from such feedstocks. Distillate fractions of suitable boiling ranges for lube base stock can be dewaxed by extraction with solvent mixtures such as methyl ethyl ketone and toluene, or methyl ethyl ketone and methyl isobutyl ketone. Because solvent extraction processes generally require large amounts of expensive solvents, alternative or supplemental methods of dewaxing have been devised.
In recent years techniques have become available for catalytic dewaxing of petroleum stocks. A process of that nature developed by British Petroleum is described in The Oil and Gas Journal dated Jan. 6, 1975, at pages 69-73. See also U.S. Pat. No. 3,668,113, incorporated herein by reference.
In U.S. Pat. No. Re. 28,398, incorporated herein by reference, is described a process for catalytic dewaxing with a catalyst comprising zeolite ZSM-5. Such process combined with catalytic hydrofinishing is described in U.S. Pat. No. 3,894,938, also incorporated herein by reference.
In U.S. Pat. No. 4,137,148 is described a process for preparing specialty oils of very low pour point and excellent stability from a waxy crude oil distillate fraction by solvent refining, catalytic dewaxing over a zeolite catalyst such as ZSM-5, and hydrotreating, under specific conditions. The entire contents of that patent are incorporated herein by reference.
Hydrocarbon lubricating oils have been obtained by a variety of processes in which high boiling fractions are contacted with hydrogen in the presence of hydrogenation-dehydrogenation catalysts at elevated temperatures and pressures. One such process is disclosed in U.S. Pat. No. 3,755,145, incorporated herein by reference, relating to catalytic lube dewaxing using a shape-selective zeolite catalyst, a large pore cracking catalyst such as clay or silica, and a hydrogenation/dehydrogenation catalyst. In U.S. Pat. No. 4,181,598, incorporated herein by reference, a lube base stock oil of high stability is produced from a wax crude oil fraction by solvent refining, catalytic dewaxing over a shape-selective zeolite and hydrotreating under specified conditions. In both processes, there is a consumption of hydrogen and lubricating oil fractions are separated from the resulting products. The separated lubricating oil fractions differ from those obtained by fractional distillation of crude oils and the like, in that they have relatively higher viscosity index values. These lubricating oil fractions suffer from the shortcoming that they are unstable when exposed to highly oxidative environments. When so exposed, sediment and lacquer formation occurs, thus lessening the commercial value of such lubricants. The instability of catalytically dewaxed lube base stocks arises from the presence of easily oxidizable olefins in the catalytically dewaxed product. Efforts to saturate these olefins by reacting them with hydrogen, i.e., hydroprocessing, have been successful in significantly reducing the olefin content. However, prior art methods of hydroprocessing result in the removal of antioxicant sulfur compounds such as thiols or sulfides from the hydroprocessed product, thus lowering its oxidation stability.
The anti-oxidation capability of sulfur compounds is known. See G. H. Denison, Jr. and P. C. Condit "Oxidation of Lubricating Oils," Industrial and Engineering Chemistry, Vol. 37, No. 11, pp 1102-1108, 1945; D. Barnard et al, "The Oxidation of Organic Sulphides, Part X: The Co-Oxidation of Sulfides and Olefins", J. Chem. Soc., pp. 5339 to 5344, 1961. Several methods have been discovered whereby sulfur in various forms is added to improve petroleum products.
U.S. Pat. No. 2,914,470 is directed to hydrorefining a petroleum oil fraction by contacting it with a catalyst in the presence of hydrogen sulfide for the purpose of increasing the life of the catalyst. Increases of hydrogen sulfide concentration employed result in decreases in the sulfur content of the product.
U.S. Pat. No. 3,972,853 relates to a process for preparing a stabilized lubricating oil resistant to oxidation which includes contacting the lubricating oil stock with a small amount of elemental sulfur (0.1 to 0.5 percent by weight) at a mild contact temperature of about 25.degree. C. to about 130.degree. C. The elemental sulfur may be added as such or else generated in situ from sulfur precursors such as H.sub.2 S or an added organosulfur compound.
U.S. Pat. No. 3,904,513 is directed to a method of improving the oxidative stability of solvent refined lube stocks. These lubricating base charge stocks are contacted with a catalyst containing nickel-molybdenum on a large pore alumina catalyst in the presence of a gas mixture of about 90% H.sub.2 and 10% H.sub.2 S under hydrofinishing conditions. Hydrofinishing under these conditions results in a product which contains significant amounts of sulfur materials which contribute to the oxidative stability of the lube stock. However, it is also known that the presence of H.sub.2 S in the hydrogen feed deleteriously affects the catalyst used in the hydrodewaxing of hydrocarbons. Excessive H.sub.2 S in the hydrogen feed results in the undersirable acceleration of of catalyst aging rate on significant increases in product pour point, see, e.g., U.S. Pat. No. 4,283,272, incorporated herein by reference.